Calculations

Measurement data is stored in three fields in Thickness Measurements:

• Readings: Stores one or more values representing thickness measurement data collected at a specific TML.
• Uncorrected Measurement: Stores the unadjusted measurement value.
• Measurement Value: Stores the final, adjusted measurement value.

Measurement data is stored in three fields in Thickness Measurements:

• Readings: Stores one or more values representing thickness measurement data collected at a specific TML.
• Uncorrected Measurement: Stores the unadjusted measurement value.
• Measurement Value: Stores the final, adjusted measurement value.

Measurement data is recorded in these fields according to the following workflow:

1. When a Thickness Measurement is created, you enter data into the Readings field.
   Note: Readings can be added by accessing the Thickness Measurement directly, using the Measurement Data Entry Workspace, or using a datalogger.
2. A calculation is performed against the values in the Readings field, and the result is used to populate the Uncorrected Measurement field automatically.
3. The Measurement Value field is populated automatically with the value in the Uncorrected Measurement field.
   Note: The Uncorrected Measurement field is meant to serve as an intermediate storage location between the Readings field and the Measurement Value field. In baseline Thickness Monitoring, the Measurement Value is simply populated with the value in the Uncorrected Measurement field. You can create custom rules to perform additional adjustment or correction calculations.

The type of calculation that is performed in step 2 is determined by the settings defined for measurement readings in the TM Admin Preferences. The baseline TM product offers three calculation options: Average, Minimum, and Maximum.

Asset Corrosion Analysis > Average Corrosion Rate

Asset Corrosion Analysis > Statistical Corrosion Rate

TML Corrosion Analysis/Corrosion Analysis Settings > Short Term Corrosion Rate

TML Corrosion Analysis > Factor Remaining Life Date

Thickness Monitoring (TM) can determine which Allowable Stress value to use for calculating T-Min and Maximum Allowable Working Pressure by looking in the Piping Stress reference table or the PV Stress reference table and finding a record where the Material Specification, Material Grade, and Design Code match exactly the values in the Thickness Measurement Location (TML).

Details

When Interpolation Is Used

When Interpolation Is Not Used

T-Min Formulas

The following table specifies the reference table that is used to retrieve or calculate the value in the Allowable Stress field in a TML record based on the value in the Component Type field.

The following table provides the list of fields in each of the aforementioned reference tables, and specifies how the fields are mapped to the fields in the TML family. The values in these fields are used to calculate the value in the Allowable Stress field.

Maximum Allowable Working Pressure is calculated for piping and pressure vessel assets and TMLs. Values in the Allowable Stress Reference Table are also used to calculate MAWP.

For assets, the MAWP is the most conservative value from all TMLs associated with the asset and is displayed in the Analysis Output section of the Analysis Overview page.

MAWP Requirements: Piping

The Piping MAWP can be calculated for the following design codes:

• B31.1
• B31.3
• B31.4
• B31.8

For the GE Digital APM system to calculate the MAWP for piping, the following values are required:

• In the Thickness Monitoring Location Record:
  • Allowable Stress.
  • Code Year.

  • Design Code.

  • Joint Factor

  • Outside Diameter.

• In the Thickness Measurement Record, the Last Measurement Value.

  -or-

  In the Thickness Monitoring Location Record, the Nominal Thickness.

• In the TML Corrosion Analysis, the Controlling Corrosion Rate.

  -or-

  In the Corrosion Analysis Settings, Minimum Corrosion Rate

• A positive value for t-actual.

MAWP Requirements: Pressure Vessel

The pressure Vessel MAWP can be calculated only for TMLs whose design code is ASME VIII DIV 1 and whose Component Type or Vessel Type is Cylindrical Shell.

For the GE Digital APM system to calculate the MAWP for pressure vessels, the following values are required:

• In the Thickness Monitoring Location Record:
  • Allowable Stress.
  • Code Year.
  • Design Code.
  • Joint Factor.
  • Inside Radius

• In the Thickness Measurement Record, the Last Measurement Value.

  -or-

  In the Thickness Monitoring Location Record, the Nominal Thickness.

• In the TML Corrosion Analysis, the Controlling Corrosion Rate.

  -or-

  In the Corrosion Analysis Settings, Minimum Corrosion Rate

• A positive value for t-actual.

MAWP Formula: Piping

The MAWP formula for Piping is calculated using the formula 2 S F t / D where:

MAWP (Pounds Force per Square Inch) = 2 * Allowable Stress * Joint Factor * t / Outside Diameter

...where:

• S = Allowable Stress (Pounds Force per Square Inch)
• F = Joint Factor

• t = t-actual - 2(C-rate * I-interval)

  ...and:

  • t-actual = The value in the Last Measurement field from the Measurement family in inches.  If this value is not available, then the Nominal Thickness from the TML is used as the t-actual value.
  • C-rate = The Controlling Corrosion Rate from the TML Corrosion Analysis in inches per year. If this value is not available, then the Minimum Corrosion Rate from the TML's Corrosion Analysis Settings is used for the C-rate value.

  • I-interval = The value in the Scheduled Next Date - Last Measurement Date field in years. If this value is not available, then the value in the Default Inspection Interval field from the TML's Corrosion Analysis Settings page is used for the I-interval value.

• D = Outside Diameter (inches)

MAWP Formula: Pressure Vessels

The MAWP formula for Pressure Vessels is calculated using the formula S F t / R + 0.6 t where:

MAWP (Pounds Force per Square Inch)= Allowable Stress * Joint Factor * t / Inside Radius + 0.6 * t

...where:

• S = Allowable Stress (Pounds Force per Square Inch)
• F = Joint Factor

• t = t-actual - 2(C-rate * I-interval)

  ...and:

  • t-actual = The value in the Last Measurement field from the Measurement family in inches.  If this value is not available, then the Nominal Thickness from the TML is used as the t-actual value.
  • C-rate = The Controlling Corrosion Rate from the TML Corrosion Analysis in inches per year. If this value is not available, then the Minimum Corrosion Rate from the TML's Corrosion Analysis Settings is used for the C-rate value.
  • I-interval = The value in the Scheduled Next Date - Last Measurement Date field in years. If this value is not available, then the value in the Default Inspection Interval field from the TML's Corrosion Analysis Settings page is used for the I-interval value.
• R = Inside Radius (inches)

Example: Piping

Consider an example, where the following values are defined in the Thickness Measurement Location record:

• Allowable Stress = 16600 Pounds Force Per Square Inch
• Joint Factor = 1
• Nominal Thickness = 2.5 inches
• Last Measurement: 0.75 inches
• Controlling Corrosion Rate = 1.2 mils/year OR 0.0012 inches
• Schedule Next Date - Last Measure Taken Date = 0 years
• Outside Diameter = 10 inches

Based on these values, the Maximum Allowable Working Pressure is calculated as:

MAWP = 2 S F t / D;

...where:

• t = t-actual - 2 (C-rate * I interval)
• t = 0.75 - 2 (0.0012*0)
• t = 0.75

MAWP = 2 * 16600 * 1 * 0.75 / 10

MAWP = 2490 Pounds Force Per Square Inch

Example: Pressure Vessel

Consider an example, where the following values are defined in the Thickness Measurement Location record:

• Allowable Stress = 16600 Pounds Force Per Square Inch
• Joint Factor = 1
• Nominal Thickness = 2.5 inches
• Last Measurement: 0.75 inches
• Controlling Corrosion Rate = 1.2 mils/year OR 0.0012 inches
• Schedule Next Date - Last Measure Taken Date = 1 year
• Inside Radius = 10 inches

Based on these values, the Maximum Allowable Working Pressure is calculated as:

MAWP = 2 S F t /R+0.6t;

...where:

• t = t-actual - 2 (C-rate * I interval)
• t= 0.75 - 2 (0.0012*1)
• t = 0.7476

MAWP = 16600 * 1 * 0.7476/ 2+0.6*0.7476(12410/ =

MAWP = 5068.20 Pounds Force Per Square Inch

Thickness Monitoring (TM) provides a set of baseline formulas that can be used for determining the Calculated T-Min value. For these calculations to be successful, inputs are needed in certain fields. These inputs come from the TMLs for which the calculations are being performed. If the required fields are empty, TM will not be able to determine the Calculated T-Min value for a given TML.

The following section details fields that must be populated in TMLs in order for Calculated T-Min values to be returned using the baseline T-Min formulas. The lists of required fields are organized according to equipment type, which is derived using the value in the Design Code fields and other fields in each TML.

Piping T-Min Calculations

This section outlines the fields that are required for TM to determine the Calculated T-Min value for TMLs with the value B31.1, B31.1 A, B31.1 B, B31.1 C, B31.3 D, B31.4, or B31.8 in the Design Code field.

The following table lists additional fields that may be required based on the value in the Design Code and Piping Formula fields.

Pressure Vessel T-Min Calculations

This section outlines the fields that are required for TM to determine the Calculated T-Min value for TMLs with the value ASME VIII DIV 1 in the Design Code field.

The following table is for TMLs with the value Inside in the PV Formula field. The table lists additional fields that may be required based on the value in the Vessel Type field.

The following table is for TMLs with the value Outside in the PV Formula field. The table lists additional fields that may be required based on the value in the Vessel Type field.

Tank T-Min Calculations

This section outlines the fields that are required for TM to determine the Calculated T-Min value for TMLs with the value API 653 in the Design Code field.

The following tables lists the required fields based on the value in the Tank Type field.

Each section contains specific information about that TML and the calculation that was performed against it.

The Calculation Status dialog box is divided into the following sections for each TML:

TML : <TML ID>

...where <TML ID> is the TML ID of the TML whose calculation details you are viewing. This section displays the following information:

• By User: The User ID of the Security User who saved the current Minimum Thickness value.
• On Date: The date on which the current Minimum Thickness value was last saved.
• Proposed T-Min Value: The most conservative (i.e., highest) T-Min value, based on the User Defined, Calculated, Nominal, and Default T-Min values.
• Proposed Source: The source that determined the Proposed T-Min Value.
• Calculated T-Min Value: The value calculated by the T-Min Calculator.

Calculated T-Min Details

This section displays the following information:

• Formula Used: The location in the Rules Library where the formula that was used to calculate the Calculated T-Min value is stored.

Nominal T-Min value

• Nominal T-Min value: The Nominal T-Min value that was retrieved using the Piping Nominal T-Min reference table.
• Values used for the lookup: The fields in the TML that were used to retrieve the Nominal T-Min value.

Default T-Min value

This section displays the value that is stored in the Default T-Min field in the Corrosion Analysis Settings record that is linked to the TML.

User Defined T-Min value

This section displays the value specified in the User Defined box for the TML in the T-Min Calculator workspace.

Records in the Piping Nominal T-Min family make up the Piping Nominal T-Min reference table, which Thickness Monitoring uses to locate the appropriate Nominal T-Min value for each Thickness Measurement Location (TML) when you calculate T-Min values. Since the baseline GE Digital APM product does not include any records in the Piping Nominal T-Min family, you must create these records manually using values from industry standards. After these records exist, when you calculate T-Min values, the GE Digital APM system will map values from fields in the TMLs to values in the corresponding fields in Piping Nominal T-Min records to find the associated Nominal T-Min value.

Details

The following table lists the default fields in the Piping Nominal T-Min family and the corresponding fields in the Thickness Measurement Location (TML). The table provides the full list of mappings for all fields in the Piping Nominal T-Min family. Each mapping with be used only if that field is selected to be used for Nominal T-Min retrieval according to the settings on the TM Admin Preferences page.

When you create a Thickness Measurement Location, the Piping section of the datasheet contains the fields in the Thickness Measurement Location family, that are used to determine the values in the Nominal Thickness and Outside Diameter fields.

The following table shows how the fields in the Pipe Properties reference table correspond to and interact with fields in the Thickness Measurement Location family.

Example: Populating Fields on the Thickness Measurement Location Datasheet

Suppose you that you are starting with a blank TML datasheet, and you select the following values in the Piping section:

• Piping Nominal Diameter -NPS field: 0.125. Based on this selection, the Piping Nominal Diameter - DN field is automatically looked up in the Pipe Properties reference table, and the field value is set to 6.
• Schedule field: 40(s).

As a result, the following fields are populated automatically in the Design General section:

• Nominal Thickness field: 0.068 (inches).
• Outside Diameter field: 0.405 (inches).

Records in the Tank T-Min Annular Ring family make up the Tank T-Min Annular Ring reference table, which GE Digital APM uses to locate the appropriate Calculated T-Min value for each Thickness Measurement Location (TML) with the value API 653 in the Design Code field and Annular Ring in the Tank Type field.

The baseline GE Digital APM product does not include any records in the Tank T-Min Annular Ring family. For Calculated T-Min values to be determined, you must create Tank T-Min Annular Ring records using values from industry standards.

Details

The Reading Tolerance setting determines how much variation is allowed between values in the Readings field. When values are entered in the Readings field, they are averaged. Then, the Reading Tolerance value is used to determine which values fall within an acceptable range of the average value. If any of the values fall outside of the acceptable range, the value in the Readings Pass/Fail field is set to Fail.

The average of these three values is 0.3233. The GE Digital APM system evaluates each individual Reading value and determines whether it falls within 0.03 inches of the average value. If all the readings are within the range, then the measurement is set to Pass. If any or the reading is outside the range, then the measurement is set to Fail.

Since one of the readings failed the Reading Tolerance evaluation, the value in the Reading Pass/Fail field is set to Fail.